1. Field of the Invention
The invention concerns a system in a press section of a paper machine for monitoring and control of the running of the press felts, which press felts are guided by means of rolls, whose axial directions are arranged so that they can be altered by means of an actuator so as to control the running of the press felts, and which system includes detector devices, by whose means the alignment stripe on the felt or felts is detected, and which system includes a computer or a corresponding logic unit, into which the monitoring signals are fed from the detectors in the system.
2. Description of the Prior Art
As is well known, the presses in paper machines comprise press rolls, which form press nips with each other, through which nips the paper web runs together with a press felt or felts. The press rolls with their axle journals and beating brackets are susceptible of oscillations, which tendency is increased by the elasticity of the press felts. In press rolls, auto-oscillations occur commonly, which are typically in a frequency range of 20 . . . 150 Hz. These oscillations produce noise and affect the quality of the paper web produced, because the oscillations, especially oscillations taking place in the nip plane, produce variations in the compression pressure in the nip. Further, the oscillations produce shrinkage and wave-like barring in the press felts and undue wear of the felts. Said barring tends to increase the so-called felt-induced oscillation of the press rolls, and when the wave-like barring in the felts becomes deeper, the amplitude of said oscillation tends to increase constantly.
Said felt-induced oscillations occur mainly at the harmonic frequencies f.sub.N =N.multidot.v/L, wherein N=an integer, v=felt velocity, and L=overall length of felt loop. When oscillations are noticed in the press section at said different "resonance frequencies" f.sub.N, it can be concluded that felt-induced oscillations are concerned, which are derived from said very felt whose length is L and velocity v.
Attempts have been made to prevent felt-induced oscillations of press rolls thereby that the running direction, i.e. the "splice", of the felt is, at suitable intervals, turned by a few angle degrees so that the felt runs over the spreader, tensioning and alignment rolls (in the following, the general denotation "guide roll" will also be used for these rolls) in slightly varying positions. Said splice turning means that the felt loop runs in a slightly diagonal position, i.e. the length of one of its edges is somewhat larger than the length of the opposite edge. This means that any transverse stripes and bars that may be formed in the felt do not run through the press nips exactly as parallel to the longitudinal directions of the nips but at a small angle diagonally, in which case, when the bar or stripe runs through the nip, it does not induce an oscillation of a detrimentally high amplitude in the press rolls.
The prior-art felt-splice turning systems have, as a rule, been based on the experience acquired by the operating personnel in practice. Manual felt guide systems require constant alertness from the operating personnel. It is often difficult to judge how frequently the axial alignment of the felt guide rolls must be changed, i.e. the "splice" be turned. Moreover, by means of the manual control systems, it cannot even be found out with certainty whether the control operation is of the right direction, i.e. whether it increases or reduces the oscillations in the press rolls. Since the experience and the professional skill of the operating personnel vary and the properties and the qualities of the press felts change, by means of manual felt control and splice-turning, a sufficiently good result has not been achieved in respect of the quality of the paper produced and in respect of the wear of the various components in the press section, such as the felts, press rolls, and their bearings.
In order to avoid the drawbacks discussed above and in view of further development of the manual control systems, some press-felt splice-turning systems have been suggested, which are to some extent automatic and/or which are based on oscillation measurements, and in respect of said systems, reference is made, by way of example, to the FI Patent Applications Nos. 891386 and 882730 (equivalent of DE 38 09 526 and DE 37 19 828.9). According to the latter application, the press-felt splice turning takes place as timed by a switch clock or as linked with the measured level of oscillation. The oscillation criterion is a band-pass filtered and rectified signal, which involves the drawback that the factual source of the oscillation at different running speeds of the machine cannot be identified.
In the splice-turning systems discussed above and in other, corresponding systems, there are several needs of development, for example, because the splice turning takes place constantly or at certain fixed time intervals, so that the splice cannot be turned optimally and in accordance with a need that varies in respect of time. For example, with a new felt there is a necessity to turn the splice more frequently. The most important drawbacks in the systems in accordance with the above FI applications are therein that it cannot be concluded with which felt the particular oscillations are in phase and that, by means of the systems, the factual running position of the felts cannot be noticed or measured.